In general, a laser system is constructed with three main parts including an energy source, a laser medium, and two or more mirrors that form an optical resonator. The energy source of a laser system is also known as the pump source, and is the part that provides energy to the laser system.
The laser medium of a laser system is also known as the gain medium, and is the major determining factor of various properties of the laser system including the wavelength of operation. The gain medium is excited by the pump source to produce a population inversion. The gain medium is also where spontaneous emission of photons and stimulated emission of photons take place that lead to the phenomenon of optical gain, or amplification. Gain media are generally made of liquids, gases, solids, or semiconductors. The solids used as gain media typically include crystals and glasses, and may be doped with an impurity such as chromium, neodymium, erbium, or titanium ions. As the gain medium is excited to emit photons, a large amount of thermal energy is generated by the gain medium. Such thermal energy needs to be removed from the gain medium, by a cooling apparatus for example, in order to prolong the lifetime of the gain medium as well as keep the laser system within normal operating parameters.
The optical resonator consists of two or more mirrors placed around the gain medium to provide feedback of the light. Light from spontaneous emission of the gain medium is reflected by the mirrors back into the gain medium, where the light may be amplified by stimulated emission. The light may be reflected from the mirrors and thus pass through the gain medium hundreds of times before exiting the gain medium. The design and alignment of the mirrors with respect to the gain medium is crucial to determining the exact operating wavelength and other properties of the laser system.
In applications where the laser system needs to be compact, one or more laser diodes may be used as the energy source given the small form factor of laser diodes. The gain media in such a laser system correspondingly tends to have small form factor as well. However, conventional metal-based cooling apparatus made of copper, aluminum or a type of metal alloy tend to suffer from corrosion and clogging of coolant channel, if liquid is used as a heat transfer medium. Additionally, at very high temperature the metal may deform if the temperature approaches the melting temperature of the metal.